\begin{tikzpicture}
    \pgfmathsetmacro{\R}{1.5}

    \tkzDefPoints{0/0/O}
    \tkzDefPoint(70:\R){A}
    \tkzDefPoint(150:\R){B}
    \tkzDefPoint(30:\R){C}
    \tkzDefPoint(330:\R){D} % BD 为直径

    \tkzDrawCircle[thick](O,A)
    \tkzDrawPolygon(A,B,C)
    \tkzDrawSegments[dashed](B,D  C,D)
    \tkzDrawPoint(O)

    \tkzMarkAngle[size=.2](B,A,C)
    \tkzMarkAngle[size=.4, arc=ll](C,D,B)
    \tkzLabelSegment[pos=.7, below](B,C){$a$}
    \tkzLabelPoints[below](O)
    \tkzAutoLabelPoints[center=O, centered, dist= .2](A,B,C)
\end{tikzpicture}

